This invention relates to a motor vehicle suspension control having a differentiator circuit for converting a vertical suspension position signal from a relative suspension position sensor between the sprung and unsprung masses to a relative suspension velocity signal useful in deriving a control signal for a suspension actuator to produce a desired suspension behavior.
Such a differentiator has several requirements which must be met. The first is that it operate as a differentiator within a certain frequency range of interest. Passenger motor vehicles typically have suspension resonances in the region of 1 Hz for the vehicle body or sprung mass and about 8-10 Hz for the wheel apparatus or unsprung mass. In addition, the frequency of greatest sensitivity of a seated occupant to vertical vibration is about 5-6 Hz. Therefore, the circuit must have the characteristics of a differentiator--that is, linearly increasing gain and a constant 90 degree phase lead--throughout a frequency range including these frequencies: for example, 0.5 to 20 Hz.
A typical textbook differentiator circuit of the type capable of producing such a response is shown as differentiator 12 of FIG. 5 in U.S. Pat. No. 4,579,366 to Doi et al, issued Apr. 1, 1986, which circuit comprises an operational amplifier OP3 with a series input capacitor C and a negative feedback resistor R6. However, the linearly increasing gain of this circuit continues past the upper limit of the frequency range of interest until a high frequency roll-off begins at a significantly higher frequency due to the inherent capacitances of the circuit. The result is an extended high gain above the frequency range of interest which tends to amplify high frequency noise.
A preferred form of suspension control for motor vehicles not shown by Doi et al comprises a digital microcomputer or other programmed digital signal processing apparatus. An analog relative suspension velocity signal must be A/D converted to a digital signal for use in such apparatus; and the digital signal processing apparatus thus samples the analog signal at a predetermined sampling frequency: for example, 1 KHz. In order to prevent aliasing distortion, the sampling frequency must be at least twice the highest frequency in the analog input signal. Thus, any component of the analog relative suspension velocity signal above the aliasing frequency of 500 Hz, which is one half the sampling frequency, must be suppressed. This means that the gain of the differentiator circuit, in combination with any additional filters, must decrease to a predetermined low value at the aliasing frequency of 500 Hz. Typically, the natural high frequency roll-off of the inherent circuit capacitance is insufficient to accomplish this task; and further circuit elements must be added to produce additional low pass filter poles and thus accelerate the roll-off with frequency.
Unfortunately, such low pass filter poles also cause accelerated phase change with frequency, which introduces group delay; and this group delay can degrade the performance of a real time suspension control system. Increasing the number of low pass filter poles increases the roll-off in gain; but it also increases the group delay. In fact, the main problem becomes a tradeoff between high frequency gain roll-off and group delay so that it is difficult to simultaneously provide (1) the required differentiator characteristic in the range of vehicle suspension resonant frequencies, (2) sufficient roll-off in gain at the aliasing frequency of A/D conversion, and (3) sufficiently low group delay in a differentiator circuit.